Bacterial endotoxins (or lipopolysaccharides) can enhance the resistance to transplantable tumors (TUR) in experimental animals. Neither the moiety of the endotoxic molecule which can enhance TUR nor the mechanisms by which this enhancement is achieved are known. The aim of this investigation is to clarify these by using a) smooth as well as mutant endotoxins, and endotoxin breakdown products, b) five tumor models and c) various modulators of immune responsiveness.